Field of Invention
This invention relates principally to manufacture of plastic articles and even more particularly relates to pneumatic conveyance and processing of plastic resin pellets, as well as other granular materials, prior to molding, extrusion, or other processing of those pellets or other granular materials into a finished or semi-finished product.
In this patent application, injection and compression molding presses and extruders are collectively referred to as “process machines.”
Description of the Prior Art
Current resin and other granular material central loading systems concerned with conveying granular material from a storage area for molding or extrusion typically include a vacuum pump or pumps and multiple receivers.
In some systems, with many receivers, several small pumps are used.
It would be less expensive to use only one, or fewer, larger pumps. However, a larger pump may draw too much air with resulting damage to the material being conveyed. While a larger pump could load several receivers at once, there is a risk that an “open” line, namely a line drawing only air, and no material, would cause the vacuum to drop too much, and no material would load. Also, when only one receiver is loading material, air velocity might be too high, again with a risk of damaging the material.
Nevertheless, in facilities that fabricate products by molding or extrusion, it is common to use such vacuum loading systems to pneumatically convey pellets of thermoplastic resin or other materials, prior to molding, extrusion, or other processing of those pellets or other materials into a finished or semi-finished product. The materials are typically purchased in 50 pound bags, 200 pound drums, or 1,000 pound containers commonly referred to as “Gaylords.”
A preferred approach for conveying plastic resin pellets and other granular materials from a storage location to a process machine, which approach is often used in larger facilities, is to install a central vacuum pump or even several vacuum pumps, connected by common vacuum lines to multiple “receivers.”
Vacuum pumps connected to the vacuum lines draw vacuum, namely air at pressure slightly below atmospheric, as the vacuum pump sucks air through the “vacuum” line. The suction moves large quantities of air which carries thermoplastic resin pellets or other granular material through the “vacuum” line.
An alternative is to use positive pressure produced by either a blower or the exhaust side of a vacuum pump. With such an approach, the positive pressure results in movement of substantial amounts of air which may be used to carry the granular material. However, the vacuum approach of drawing or sucking granular material through the system conduits is preferable to the positive pressure approach of pushing the material granules through the system conduits.
In practice, vacuum pumps are preferred and vacuum lines are desirable in part because power requirements to create the required vacuum necessary to draw granular materials through the lines are lower than the power requirements if the material granules are pushed through the lines by a blower or by the exhaust side of a vacuum pump. When vacuum is used, the static pressure within the line may be not much less than atmospheric. When positive pressure is used, the dynamic pressure of the air flowing through the line must be relatively high in order to move an adequate quantity of granular material.
As used herein, and in light of the foregoing explanation, the terms “vacuum pump” and “blower” are used interchangeably.
When one or more central vacuum pumps are connected to multiple receivers, a receiver is typically located over each temporary storage hopper, in which the plastic resin pellets or other granular material is temporarily stored before being molded, extruded, or otherwise processed. A temporary storage hopper is typically associated with each process machine.
In current practice, the receiver is connected by a control wire to a central control system. The control system works by selectively opening a vacuum valve located in each receiver, allowing one or several vacuum pumps to work in sequence drawing “vacuum”, i.e. below atmospheric pressure air, to carry the pellets or other material granules among and to multiple receivers as individual ones of the receivers, positioned over individual hoppers associated with the individual process machines, require additional plastic resin pellets or granules of other material. The receiver for a given hopper-process machine combination is actuated by opening the vacuum valve located in or near the receiver, causing the receiver to supply plastic resin pellets or granules of other material by gravity feed into the hopper from where the pellets or other material granules may be fed further downwardly by gravity into the associated process machine.
Large, high capacity industrial vacuum pumps are reliable and are suited to heavy duty industrial use. Large high capacity vacuum pumps allow long conveying distances for the plastic resin pellets and other granular materials. Currently available large capacity vacuum pumps permit plastic resin pellets and other granular materials that are similar in size and density to be conveyed over distances of 200 feet or more using vacuum drawn by the pump. Use of such high capacity vacuum pumps results in a rush of below atmospheric pressure air through the line, carrying the plastic resin pellets or other granular materials over a long distance.
Operators of manufacturing facilities prefer to buy plastic resin pellets and other necessary granular materials in bulk, in rail cars or tanker trucks. Bulk purchases result in cost savings. Such materials delivered in bulk are typically pumped into large silos for storage. In a large manufacturing facility, the distance from a plastic resin pellet or other material storage silo to a process machine may be several hundred feet, or more. Accordingly, when plastic resin pellets or other granular materials are purchased in bulk, a central vacuum-powered conveying system, powered by one or more large, high capacity industrial vacuum pumps, is a necessity.
Typically, large central plastic resin pellet and other similar granular material conveying systems have one or more vacuum pumps, each typically from 5 to 20 horsepower. These central systems include central controls connected by wire to each receiver associated with each process machine in the facility. Typically eight, sixteen, thirty-two or sixty-four receivers, each associated with a process machine, may be connected to and served by the central vacuum conveying system. Of course, the higher the number of receivers served by the system, the higher the cost.
A factor to be considered in designing such a system is the speed of the plastic resin pellets or other material granules as they flow through a conduit as the pellets or granules are carried by the moving air stream drawn by the vacuum pump. If air flow is too slow, the pellets or other granules fall out of the air stream and rest on the bottom of the conduit, with resulting risk of clogging the conduit. If air flow is too fast, the pellets or other granules can skid along the conduit surface. In such case, harder, more brittle plastic resin pellets and other granular materials may be damaged, resulting in dust within the conduit, which when drawn into the vacuum pump can damage the vacuum pump and render the system inoperative. Softer plastic resin pellets and other soft granular materials heat up and can melt from friction when contacting the conduit interior surface. In the case of plastic resin pellets, this results in “angel hair”—long, wispy-thin strands of plastic film which eventually clog the conduit and cause the system to shut down.
For these reasons, pneumatic plastic resin pellets and other granular material conveying systems must be designed to produce desired, reasonable conveying speeds for the conveyed materials.
Currently, conveying speed of the plastic resin pellets and other granular material is most often controlled by controlling air flow, measured in cubic feet per minute, and varying the desired and designed cubic feet per minute based on conduit diameter, with a larger diameter conduit requiring more cubic feet per minute of air flow to maintain proper air flow speed through the conduit. Controlling air flow, measured in cubic feet per minute, is conventionally done by properly specifying the vacuum pump by capacity and, in some cases, by varying speed of the vacuum pump as the vacuum pump draws the air in a “vacuum” condition through the conduit, carrying plastic resin pellets or other material granules in the moving, below atmospheric pressure air. Controlling cubic feet per minute of air flow is an indirect way of controlling plastic resin pellet or other material granule speed as the pellets or other granules flow through a conduit of a given diameter.
Typically, a 2 inch diameter conduit requires about 60 cubic feet per minute of air flow to convey typical plastic resin pellets or other granular material of similar size and density characteristics. A 2½ inch diameter conduit typically requires about 100 cubic feet per minute of air flow to convey typical plastic resin pellets or other granular material. To achieve these desired air volume flow rates, a conventional designer must carefully match the horsepower of a vacuum pump, which has a given cubic feet of air per minute rating, to a selected size conduit, taking into consideration the average distance the plastic resin pellets or other material granules must be conveyed through the conduit from a storage silo to a receiver or loader. If this results in selection of a 5 horsepower blower/vacuum pump, then a given facility may require several such blowers/vacuum pumps, with each blower/vacuum pump supplying only a selected number of receivers.
A single plastic resin molding or extruding facility or another type of granular material processing facility might theoretically require a 20 horsepower blower and the corresponding cubic feet per minute capability for conveyance provided by the single blower to meet the total conveying requirements for plastic resin pellets or other material granules throughout the facility. However, a single twenty horsepower blower would result in far too high a conveying speed for the plastic resin pellets or other material granules through any reasonable size conduit. As a result, the conveying system for the plastic resin pellets or other granular material in a large facility is necessarily divided and powered by three or four smaller blowers, resulting in three or four different, separate systems for conveyance of plastic resin pellets or other granular material. Sometimes several blowers are connected to a single set of receivers, with one or more of the extra blowers turning “on” only when required to furnish the required extra cubic feet per minute of air flow. This is controlled by a central station monitoring all receivers and all blowers, with the central station being programmed to maintain all of the hoppers associated with the process machines in a full condition, wherever those hoppers are located throughout the facility.
Even with careful planning and design, results achieved by such pneumatic plastic resin pellet or other granular material conveying systems are not consistent. Air flow speed and cubic feet per minute capacity of blowers often vary and are outside of selected design and specification values.